1. Field of the Invention
This invention relates generally to the field of disc drive data storage devices and more particularly, but not by way of limitation, to an improved latch assembly for selectively locking the read/write heads of a disc drive in a fixed location.
2. Brief Description of the Prior Art
Disc drives of the type known as "Winchester" disc drives are well known in the industry. These disc drives include a number of rigid discs mounted for rotation on a spindle motor. The surface of the discs is coated with a magnetically permeable medium on which digital information is recorded and from which this information is later retrieved. This information is usually recorded on the discs in a number of circular, concentric data tracks.
Data recording and retrieval of information from the discs is accomplished with read/write heads, each of which is typically associated with, and arrayed in close proximity to, one surface of a disc. These read/write heads are commonly in the form of an interrupted ferrous ring wrapped with wires or an analogous form fabricated using thin-film deposition techniques. To record data, controlled timed current pulses are applied to the wires, inducing magnetic flux in the ferrous ring, or core. At the point where the core is interrupted, or the gap, this flux extends beyond the limits of the core to magnetize the adjacent recording medium. Retrieval of this information is accomplished when the spinning disc brings the previously recorded data under the gap of the head. This induces current spikes in the wires which are amplified and passed to electronic read data logic.
The read/write head is integrated into a slider which incorporates an arrangement of ski-like features on the side of the slider body closest to the disc surface. Before power is applied to the disc drive, the slider is in contact with the disc. Upon the application of power, the discs begin to spin, dragging along a thin layer of air. At some point in the acceleration of the disc as the disc is increased in rotational velocity to achieve to operational speed, the ski-like features on the slider interact with this layer of air to fly the heads above the disc surface on an air bearing. This air bearing serves to minimize wear and lessen the power needed to move the heads across the disc surface.
In order for the single head associated with each disc surface to access all of the data tracks, the read/write heads are mounted on an actuator assembly which includes a motor under control of additional logic. This actuator mechanism moves the heads from track to track in a controlled manner.
In the earliest rigid disc drives employed in personal computers, the motor used in the actuator assembly to move the heads was typically a stepper motor. The design of stepper motors results in a significant magnetic detent which tends to hold the stepper motor in the position to which it was last driven. This feature was utilized to hold the heads in whatever location they occupied upon loss of power and is known as a "random parking" scheme.
The need for faster access to data led the industry to employ voice coil motors (VCMs) in the actuator assembly. Voice coil motors employ the same technology used in loud speakers, that is, an electrical coil and a permanent magnet whose interacting magnetic flux fields result in rapid, controllable movement. While the use of VCMs has resulted in very rapid data access times, often less then ten milliseconds, the VCM has no inherent detent, or holding capability, when power is removed. This fact has given rise to the standard practice of moving the read/write heads to a designated "parking zone" when the loss of power is detected, and mechanically latching the actuator assembly in the parking zone until power is restored.
Most often the parking zone is at an inner area of the discs closely adjacent the spindle motor, that is, in an area where no data are recorded, in order to minimize the starting torque requirement of the spindle motor. Because the heads rest in contact with the disc surfaces when power is removed, the parking zone is sometimes referred to as a "landing zone". Latching the actuator assembly when the read/write heads are at the landing zone prevents relative motion of the heads and discs when in contact in order to minimize the risk of damage to either of these critical components.
Latching assemblies have been designed in many forms. Some have employed spring biased arms which contact specially formed striker surfaces on the moving actuator when the actuator moves the heads to the parking zone. Typically, the spring bias must then be overcome to unlatch the actuator at power on. The methods used to overcome this spring bias have included solenoids, and air vanes which are moved by air currents formed when the discs begin to spin.
Other latch assemblies have been designed using permanent magnets fixed on a stationary portion of the disc drive which contact a magnetizable striker surface on the moving actuator. In such latch assemblies the actuator motor is frequently employed to overcome the latching force when power is restored. Such an arrangement requires a careful compromise between the latching force of the magnet and the available power of the actuator motor. If the magnetic latching force is too great, the motor may not be able to overcome it, and if the latching force is too small, the actuator may not stay latched under specified mechanical shock loads. Furthermore, this type of latch can introduce a bias to the actuator when the disc drive is accessing the data tracks closest to the parking zone.
Mechanical latches which use solenoids have also been employed to unlatch the actuator. Generally, such mechanical latches require that power be continuously supplied to the solenoid to keep the latch open during normal operation. Industry demands for low-power disc drives normally preclude the use of this type of latching assembly.
Many of the prior art latches require that precise adjustments be made when the latch is installed into the disc drive to insure proper operation. This precision adjustment not only adds to the time required to install the latch, but a failure to adjust the latch properly can result in improper operation of the latch, leading to costly rework.
Clearly, the need exists for a latch assembly that does not introduce any mechanical bias to the operation of the actuator assembly and that does not require power to maintain the latch assembly in an open, or unlatched, condition during normal operation.